The field of this invention relates to selecting cells for assembly into lithium-ion battery packs.
Lithium ion secondary cells have been incorporated in numerous portable electronic devices, e.g., cellular phones, laptop computers, and video recorders, because lithium ion secondary cells provide superior running voltage and increased energy density. Due to these outstanding characteristics, lithium-ion secondary cells are also being incorporated into medical applications.
When lithium-ion secondary cells are used, they are normally configured in parallel/series combination to provide the required running time and power needed to operate the associated device. The problem is that when lithium-ion cells are cycled outside of a carefully controlled voltage range, they can experience safety problems. To diminish these problems, a voltage supervisor is used to monitor cell voltages within a pack of lithium cells, and to interrupt current into or out of the pack should the voltage on any one cell exceed or fall below preset voltage limits.
Even within a safe operating voltage range, differences in performance of individual lithium-ion cells are observed at different voltage limits. To resolve this problem, others have developed circuitry to artificially balance cells within a battery pack. Such alternative embodiments are disclosed in U.S. Pat. Nos. 5,519,563 to Higashijima et al., 5,530,336 to Eguchi et al., 5,504,415 to Podrazhansky et al., 5,886,502 to Higashijima and 5,998,967 to Umeki et al. These patents balance the circuitries by bringing the voltages of different batteries within the pack to the same terminal voltage at the end of the charge.
When a lithium ion battery is charged, it is first charged at constant current, and then at constant voltage. Under constant current charge, the cell is charged at a set current proportional to the related capacity of the cell until a desired voltage is achieved. If multiple cells in series are being charged, the entire series stack is charged at a constant current until a desired constant voltage, equal to the number of cells in the stack multiplied by the desired single cell voltage, is achieved. The cell or series stack is then held at constant voltage until the charge current decays to a preset lower limit.
During the charge of multiple lithium cells in series, the same current must pass through all cell elements. If one of the series cell elements is more resistive than the others, it will generate a higher internal voltage on charge, and surpass the desired terminal voltage limit sooner than the less resistive cells. When the entire series string passes into constant voltage charging mode, the cell will be at a higher voltage than the other cell elements in the string. The other series cell elements will be at a lower voltage. The reversible capacity of the higher voltage cell will degrade due to exposure to this higher voltage. If no cell balancing is employed and as the internal resistance of all the cells increases with cycling, the higher voltage cell or cells may eventually exceed the supervisory over voltage limit, shutting down the pack before it is fully charged. Furthermore, the more resistive cell will experience a greater voltage drop on discharge. Again, if no cell balancing is employed, this cell may eventually fall below the preset supervisory under voltage limit, shutting down the pack before the less resistive cells are fully discharged.
The present invention relates to a battery pack having at least a first non-aqueous secondary cell and a second non-aqueous secondary cell. Each non-aqueous secondary cell has a discharge capacity and an internal resistance to direct charge current. To diminish and alleviate problems associated with conventional battery packs, the present invention has the internal resistance to direct charge current and the discharge capacity of each non-aqueous secondary cell substantially matched to each other. Thereby, the battery packs have longer running voltage and increased energy capacity after cycling.